In a variety of commercial, industrial, and academic endeavors the accurate and precise measurement of distances is vital to accomplishing critical tasks. One metrology technique that is widely used is optical interferometry. Optical interferometric devices can be used to measure the distance to an object or surface or to measure the displacement of that object or surface over time. Such optical interferometric devices can be integrated into objects with large scale volumes to measure coordinates within that volume, particularly where a cooperative target (i.e., a target that reflect light back to a detector) is present. When no cooperative target is available, optical interferometric devices can scan a target or surface to gather information about the target or surface.
With the cooperative target, a wavelength-stabilized laser interferometer, commonly referred to as a “laser tracker,” has been used for displacement measurements. Commercially available laser trackers can determine displacement by continuously counting incremental displacement, which can be determined from counting longitudinal interference fringes. Since the fringe needs to be continuously counted, the laser beam from the laser tracker must be subject to a beam tracking method that does not break the beam from the cooperative target. Such an arrangement is possible when the cooperative target is a corner cube retroreflective target with a spherical back surface for which the inner center is aligned with the center of the reflector. Such a corner cube is commonly referred to as a spherically mounted retroreflector (“SMR”). The resolution of displacement measurements are a few folds of wavelength. An optical interferometer with a stabilized helium-neon laser could provide the resolution of half the wavelength (λ/2=316.5 nm). The accuracy of the measurement depends upon the stability of the wavelength and the sensitivity of the sensors, i.e., how the accuracy of the sensor is affected by environmental factors, such as temperature, pressure, and humidity. Thus, the accuracy of distances measured with optical interferometer is reliable so long as the wavelength of light is stable and traceable with a reference light source.
While the described SMR tracking interferometer with a wavelength-stabilized laser that is arranged to account for environmental conditions can provide relatively high resolution and high-speed distance measurements, to measure the absolute distance to a non-cooperative target or scan along a surface of a three-dimensional object, typically requires a broadband or wavelength-tunable light source. Frequency modulated coherent laser radar (“FM-LR”) is one broadband interferometry technique that uses a wavelength-tunable light source. FIG. 1 schematically illustrates an exemplary FM-LR 10 known in the prior art.
FM-LR is generally analogous to conventional radar techniques, but is applied to the optical domain using coherence of light. In a radar system, a sweep of a radio frequency (“RF”) wave is mixed with a local oscillator, which serves as a reference for the sweep. The beating frequency, defined by the frequency difference between the receiving signal and the local oscillator, provides the inverse of the time-of-flight of the receiving RF wave, which can be translated linearly into distance. In FM-LR, the optical frequency is swept in time for the sweep, where the local oscillator is created by splitting the swept source beam. The light from the measurement arm in the interferometer is combined with the local oscillator from the reference arm. If the optical path lengths from both the measurement arm and the reference arm are within the coherence length of the light source, interference will be realized in the wavelength sweep, which is proportional to the inverse of the swept wavelength linewidth.
In an example of a basic implementation, the Fourier transformation of the acquired interferogram, calibrated in optical frequency coordinate, will provide a point spread function (“PSF”) of the light reflected from a target surface. For the PSF, distance information can be determined by determining the peak of the PSF. The precision of the distance determination relies on the algorithm used along with the signal-to-noise ratio (“SNR”). In general, with an SNR of 0 dB (i.e., the noise amplitude is the same as the signal amplitude), the distance repeatability in terms of the standard deviation is typically about 50 folds of the full width at half maximum (“FWHM”) of the PSF.
When FM-LR is applied to non-contact volumetric metrology, typically a fast tunable light source is used. The frequency of the light source can be either externally modulated with an acousto-optic tunable filter or directly modulated by a driving current modulation. The frequency modulation bandwidth can be as high or higher than 50 GHz depending upon the frequency swept speed. The bandwidth is inversely proportional to the width of the PSF. For example, the 50 GHz band for a rectangular frequency sweep corresponds to the FWHM of the PSF of 4 mm. For direct current modulation, the linewidth, defined by the FWHM of a Gaussian spectral density of a distributed feedback laser diode could be as narrow as 5 MHz, which can be translated into a vacuum-space coherence length of 26.4 meters. At 0 dB SNR, the repeatability in standard deviation can be about 0.04 mm.
The FM-LR technique is useful because of its inherent background ambient light rejection, high sensitivity, and high resolution in a very long distance measurement range. However, due to the wave length tuning mechanism, the tuning speed needs to be significantly low in order to facilitate scanning of a three-dimensional surface of an object. For example, it may be possible to obtain sweep frequency up to 5 kHz with 50 GHz sweep bandwidth, but it would be difficult to obtain higher without compromising the resolution.
Another broadband interferometry technique known in the art is spectral domain reflectometry. FIG. 2 schematically illustrates a system 20 for facilitating a spectral domain reflectometry technique. The spectral domain reflectometry technique is based on the measurement of the interference pattern between light that is emitted from a broadband source 22 and split between a measurement arm 24 and a reference arm 26 of the interferometer. As is illustrated in FIG. 2, a portion of the light emitted from the broadband source 22 is transmitted through the measurement arm 24 of the interferometer to a target 28 to be characterized, where the light is reflected off the target 28 and back through the measurement arm 24. Another portion of the light emitted from the broadband source 22 is transmitted through the reference arm 26 of the interferometer to a mirror 30, where it is reflected back through the reference arm 26. The light reflected back through the reference arm 26 is a coherent reference light, i.e., a local oscillator. The light returned from the measurement arm 24 and a reference arm 26 of the interferometer is combined and interference is measured in the frequency or wavelength domain. In order to acquire the spectral interference, which corresponds to the beating frequency information between the light returned from the measurement arm 24 and a reference arm 26 of the interferometer (the local oscillator), the method acquires the spectrum directly in the spectral domain instead of sweeping a narrow line wavelength laser. High resolution spectrometer is used for acquiring the spectrogram. A high speed line scan camera can be used for high speed measurement. The measured spectral domain information is converted into the desired length domain information by use of discrete Fourier transformation, which is calculated with signal processors 32.
Currently, affordable line-scan cameras can ramp the measurement speed up to 100,000 lines per second. Since the source does not need to be swept in optical frequency, a broadband light source, such as a light emitting diode or a super luminescent laser diode, can be used. The distance range is limited by the spectrometer resolution. For example, a high resolution of 0.06 nm at 850 nm corresponds to the coherent length of 5.3 mm in a vacuum. With a light source with 40 nm FWHM Gaussian spectrum, one can achieve 8 μm FWHM of the PSF. Because the PSF is narrow enough to determine nano-scale features, this technique is used, for example, in semiconductor parts inspection. However, due to the spectral resolution limit, this technique is not feasible for high precision volumetric measurement in metrology for industrial applications.
The aforementioned interferometers and techniques for metrology have limitations. For the prior art interferometers and techniques, range detection limits the measurement ranges based on the coherence length of the interferometer system. Such interferometers and techniques can cover either a very long range with low speed (5,000 measurements per second) and moderate repeatability (0.04 mm) for scanning, or a very short range (5.3 mm) with high speed (80 kHz) and high repeatability (80 nm), but not both. These interferometers and techniques are not suitable for covering mid-range (from about 0.1 meter to a few meters) with high speed (i.e., greater than 50,000 measurements per second) and repeatability better than 0.001 mm.
In the U.S. Pat. No. 8,094,292, titled “Cross-chirped Interferometry System and Method for Light Detection and Ranging,” issued to a co-Applicant, the disclosure of which is incorporated herein by reference, an interferometric system and method are described for obtaining high-speed, high-precision and high-sensitivity time-of-flight optical range finding or position identification, which allows a direct time-of-flight to spectrum mapping to achieve spectral domain acquisition for the time-of-flight detection.